The present invention relates to a breakerless ignition system for an internal combustion engine wherein a semiconductor switch is adapted to suddenly change the primary current of an ignition coil, thereby inducing a high voltage in the secondary.
It is a general practice to advance the ignition angle of an internal combustion engine as the rotational speed of the engine is increased. But, it is sometimes desirable to retard the ignition angle when the engine speed exceeds a certain value. For instance, it may be desirable that the ignition angle of a four-cycle engine is substantially retarded when the engine speed exceeds a predetermined value so that overrunning of the engine is prevented. It may also be desirable that the ignition angle of a two-cycle engine is substantially retarded above a certain engine speed in order to increase the engine output power. A conventional ignition system adapted to meet the aforesaid requirement comprises a signal coil producing an AC signal whose positive half is applied to a semiconductor switch adapted to be turned on to conduct the primary current of an ignition coil and means for short-circuiting the negative half of the AC signal, when the engine speed exceeds a predetermined value, to retard commencement of the positive half cycle of the AC signal. Another conventional ignition system comprises an AC signal source similar to the above-mentioned one and means for bypassing part of the positive half of the AC signal to reduce the magnitude of the signal, when the engine speed exceeds a predetermined value, so that the instantaneous value of the AC signal is made to lag in reaching a level sufficient to turn on the semiconductor switch. These conventional systems are however unsatisfactory because the "angle of lag" obtained is insufficient for some applications.